This invention relates to grain-refined, gold free palladium-based dental alloys and, in particular, to grain-refined alloys for use in porcelain-fused-to-metal restorations.
Porcelain-fused-to-metal restorations consist of a metallic sub-structure coated with a veneer of porcelain. Over the years various alloys have been proposed for the sub-structure of these restorations. Many of the early alloys used gold with some platinum or palladium as the main alloy ingredients. However, with the increases and fluctuations in the price of gold and platinum in recent years, other alloys have come to play major roles in this area. One series of alloys which has gained general acceptance is based on nickel, chromium and beryllium as the main ingredients. Another series of alloys, with which this invention is concerned, is based on palladium as the dominant element.
Alloys suitability for use in porcelain-fused-to-metal restorations must satisfy a plurality of demanding conditions imposed both by the marketplace and by the physical and chemical requirements applicable to alloys for use in dental restorations. With regard to the marketplace demands, the alloy should have as low a price as possible. Specifically, it is important to avoid, if possible, the inclusion of gold in the alloy because of both the high price of this element and the essentially daily fluctuations in its price.
With regard to physical and chemical characteristics, the alloy should have a coefficient of thermal expansion such that the porcelain is under compression in the finished restoration. Further, during the porcelain firing process, the alloy must form a suitable protective oxide. Also, the alloy should have a high melting temperature so that castings made from the alloy will retain their shape during the porcelain firing process.
Of primary importance is the grain structure of the alloy. If the alloy has a good grain structure, it will have high elongation, tensile strength and toughness. These properties are important in avoiding "hot tearing" and in providing a casting with good burnishability.